The focus of the application is centered on recently discovered process of MHC class I-mediated peripheral selection of CD84 T cells. Peripheral selection is of paramount importance for: 1) maintaining the peripheral CD8+ T cell pool; 2) modifying the available peripheral CD8+ T cell repertoire generated in the thymus; and 3) possibly determining the length of a life of CD8+ T cells in the periphery. Uncovering the laws governing the process of peripheral selection will open enormous possibilities of manipulating the peripheral T cell repertoire. (Specific Aim 1) To determine whether the potential to promote peripheral selection is cell-type specific, selective expression of MHC class 1 on fibroblasts or dendritic cells will be achieved by making transgenic mice expressing the TAP 1 gene under the fibroblast-1 (FSP-1) or dendritic cell- (CD11c) specific promoters, crossing the transgenic mice to the TAP1-deficient background, and testing the ability of fibroblasts and dendritic cells to promote peripheral selection. (Specific Aim 2) To determine whether the ligands involved in the peripheral selection and positive thymic selection are identical MHC (miss)matching fetal liver and thymic epithelium will be grafted to observe the effects of matching MHC molecules in the thymus and periphery on repopulation of periphery by CD8+ T cells. In the second approach, it will be tested whether candidate self MHC-associated peptides that induce positive thymic selection of particular TCR transgenic thymocytes can also induce peripheral selection of the same TCR transgenic CD8+ cells. (Specific Aim 3) To identify true self peptides that induce peripheral selection, gene knock-out mouse strains will be immunized to produce CD8+ T cells specific for peptides derived from the missing gene. These CD8+ T cells will be used as a tool for detection of peptide presentation in wild-type mice. An alternative strategy will consist of mass spectrometry analysis of peptides eluted from immunoprecipitated MHC class I molecules. Detailed knowledge gained from the proposed experiments will be valuable for vaccine development and immunomodulation therapy in organ transplantation, autoimmune diseases and cancer.